A known water cooled internal combustion engine includes a single passage forming member (water outlet) internally defining a branch passage for distributing the cooling water that has passed through a water jacket formed in an engine main body to passages leading to a radiator, a heater core and an ATF warmer, and fitted with a thermostat for switching the supply of the cooling water to the radiator depending on the temperature of the cooling water, and a water temperature sensor for detecting the temperature of the cooling water that has passed through the engine main body. See JP2013-108429A, for instance. The use of the passage forming member contributes to the simplification and the size-reduction of the piping structure for the cooling water passages.
When the engine is equipped with a supercharger (typically consisting of a turbocharger) provided with a water jacket, the cooling water that has passed through the supercharger may be returned to the passage forming member with the aim of simplifying the piping structure. However, the cooling water that has just passed through the supercharger may have a substantially higher temperature than the cooling water that has passed through the engine main body. Therefore, if the cooling water that has passed through the supercharger is returned to a part adjacent to the water temperature sensor intended for measuring the temperature of the cooling water that has just passed through the engine main body, the water temperature sensor is unable to detect the correct temperature of the cooling water. Also, the cooling water that has passed through the supercharger may cause an undesired temperature rise in the heater core or the ATF warmer.